1. Field of the Invention
This invention relates to a keyboard musical instrument and, more particularly, to a keyboard musical instrument having keys regulated with key balance pieces.
2. Description of the Related Art
The keyboard musical instruments are categorized in three groups. The first group is an electric or electronic keyboard musical instrument, and the second group is an acoustic keyboard musical instrument. Acoustic pianos, i.e., grand pianos and upright pianos are typical examples of the acoustic keyboard musical instrument. The third group is a compromise between the electric/electronic keyboard musical instrument and the acoustic keyboard musical instrument. A silent piano is an example of the composite keyboard musical instrument between an acoustic piano and an electronic keyboard. The user has an option between acoustic piano tones and electronic tones. This means that the user can perform a passage through the acoustic piano tones or electronic tones.
In any sort of keyboard musical instrument, keys are indispensable component parts of the keyboard musical instrument, and serves as an interface between the keyboard musical instrument and users. The users specify the pitches of the tones to be produced through the keys. The acoustic keyboard musical instruments such as pianos give unique key-touch to the players, and the key-touch on the electronic keyboards is different from that of the acoustic keyboard musical instrument. Since the players are familiar with the key-touch on their keyboard musical instruments, the players, who usually play on the acoustic pianos, feel the keys of the electronic keyboards unfamiliar, and the players, who finger pieces of music on the electronic keyboard, feel the keys of the acoustic pianos strange. Professional pianists discriminate the key-touch of their own pianos from the key-touch of other pianos.
One of the factors of the key-touch is the appropriate difference between moment of force and the counter moment of force exerted on the respective keys. Another factor of the key-touch is the largeness of inertial of the respective keys. The keys are put on the balance rail so that the balance rail gives the fulcrums to the keys. Each key supports the action unit at the rear portion thereof, and exerts moment on the key. On the other hand, key balance pieces are, by way of example, embedded in the front portions of the keys in grand pianos, and exert the counter moment on the key. The key balance pieces are usually embedded in the rear portions of the keys in upright pianos. The moment is larger than the counter moment at the rest position so that the front portion floats over the key bed. When a pianist depresses the front portion, the front portion is sunk. The heavier the key balance pieces, the lighter the static key-touch. On the contrary, the heavier the key balance pieces, the heavier the dynamic key-touch. Thus, both of the differences between the moment and the counter moment and the largeness of moment of inertial have the influence on the key-touch.
FIG. 1 shows a typical example of the essential parts of the acoustic piano. In the following description term “front” is indicative of a position closer to a pianist, who sits on a stool for fingering, than a “rear” position. Term “fore-and-aft” direction is indicative of a virtual line between a front position and a corresponding rear position, and term “lateral” modifies the direction perpendicular to the fore-and-aft direction.
A keyboard 1 is mounted on a key bed 2, and a front rail 3a, a balance rail 3b and a back rail laterally extend on a key frame 4. Black and white keys 5a and 5b are put in parallel on the balance rail 3b, and extend in the fore-and-aft direction. The balance rail 3b gives the fulcrums 3d to the black and white keys 5a/5b so that the black and white keys 5a/5b are rotatable about the balance rail 3b. The front portions of the white keys 5b are covered with thin decorative plates 5c made of synthetic resin.
Capstan buttons 6 project from the rear portions of the black/white keys 5a/5b, and is held in contact with action units 7. The action units 7 are rotatably connected to a whippen rail 8, which laterally extends over the rear portions of the arrays of black and white keys 5a/5b. The whippen rail 8 is supported by action brackets 9 on the key frame 4. A shank flange rail 10 is further supported by the action brackets 9, and laterally extends over the array of black and white keys 5a/5b. Hammers 11 are rotatably connected to the shank flange rail 10. The action units 7 are functionally connected to the hammers 11, and receive the weight of the associated hammers 11. When a pianist depresses a black/white key 5a/5b, the associated action unit 7 is actuated so as to drive the hammer 11 for rotation. The jack of the action unit 7 escapes from the hammer 11, and the hammer starts the free ration. The hammer 11 strikes an associated string 12 at the end of the free rotation, and returns onto the action unit 7.
The weight of the hammer 11 and action unit 7 is applied through the capstan screw 6 to the rear portion of the associated black/white key 5a/5b, and, accordingly, the moment is exerted on the black/white key 5b in the clockwise direction. In order to partially cancel the moment, counter moment is exerted on the black/white key 5a/5b, and key balance pieces 5d are embedded in the front portion of the black/white key 5a/5b for the counter moment. The counter moment is smaller than the moment so that the front portion floats over the front rail 3a. 
The key balance pieces 5d have a generally cylindrical shape, and both ends are exposed to the side surfaces of the black/white key 5a/5b. The key balance pieces 5d are of the order of 10 millimeters in diameter. The key balance pieces 5d are also embedded in the other black/white keys 5a/5b. The key balance pieces 5d are made of lead. The key balance pieces 5d are embedded in the black/white keys 5a/5b as follows. First, through-holes 5e are formed in the front portions of the black and white keys 5a/5b (see FIG. 2). Cylindrical lead pieces are prepared, and have a diameter less than the diameter of the through-holes 5e. The cylindrical lead pieces are smoothly inserted into the through-holes 5e. A pair of bits 12 is pressed against the exposed surfaces of each cylindrical lead piece. The cylindrical lead piece is plastically deformed, and both end portions are radially spread. As a result, the deformed end portions 5f are tightly fit to the inner surfaces of the black/white key 5a/5b. Thus, the key lead pieces 5d are anchored to the associated black/white key 5a/5b by means of the deformed end portions 5f. 
The first reason why the lead is used is that the lead has the large specific gravity. The specific gravity of the lead is 11.34, and is one of the heaviest industrial metals. This means that small lead pieces give rise to large moment, and small space such as the narrow through-holes 5e are merely required for the small lead pieces. The key balance pieces 5d of lead can exert large counter moment on the black and white keys 5a/5b. In other words, a tuner can adjust the black/white key 5a/5b to the most desirable key-touch between the light key-touch and the heavy key-touch.
Another reason why the lead is used is that the lead is rich in plasticity. As described hereinbefore, the key balance pieces 5d are anchored to the associated black/white key 5a/5b through the plastically deforming process. If the cylindrical balance pieces are made of hard metal, large force is to be exerted on the both end portions, and the deformed end portions are strongly pressed against the inner surfaces. The black/white keys 5a/5b are made of wood so that the wooden key 5a/5b are liable to be broken. Moreover, the hard metal pieces are less fit to the inner surfaces of the black/white key 5a/5b, and tend to be dropped out.
Yet another reason is that the lead is economical. Although gold and platinum are large in specific gravity and rich in plasticity, they are so expensive that the people can not purchase the acoustic piano. The lead is not expensive, and the manufacturer reduces the production cost of the acoustic piano.
Thus, the key balance pieces 5d of lead are preferable for the wooden keys 5a/5b. However, the lead is detrimental to health, and contaminates the environment. Several alternate materials have been proposed.
One of the alternate materials is disclosed in Japan Patent Application laid-open 2001-142454. The key balance pieces disclosed in the Japan Patent Application laid-open are made of a sort of composite material. The composite material contains resilient material and non-lead metal. The resilient material is mixed with the non-lead metal, and the composite material is shaped in the cylindrical configuration. The resilient material enhances the elasticity of the composite material.
The key balance pieces of the composite material are embedded in the key as follows. First, the through-holes are formed in the wooded key, and cylindrical pieces are tightly inserted into the through-holes. The cylindrical pieces are pressed against the inner surfaces of the through-holes by virtue of the elasticity, the key balance pieces are anchored to the wooden key. Namely, the key balance pieces of the composite material are embedded in the wooden key as similar to the above-described prior art keys 5a/5b. 
Thus, the black/white keys 5a/5b only rely on the elasticity of the composite material. However, the through-holes are not always the adjusted to the target diameters, and the wooded keys tend to be shrunk for a long service time. In case where the through-holes have narrow through-holes, the wooden keys are liable to be cracked. On the other hand, if the through-holes are too wide, the elastic force is insufficient to keep the cylindrical pieces in the through-holes. When a pianist depresses the key, the key balance pieces chatter in the holes. If the looseness is serious, the key balance piece is dripped out. Thus, a problem is encountered in the key balance pieces disclosed in Japan Patent Application laid-open No. 2001-142454 in that the key balance pieces are not tightly fit into the keys.
Another key balance piece is disclosed in Japan Patent Application laid-open No. 2001-147685. The key balance piece disclosed in the Japan Patent Application laid-open is a combination between a tubular member and a rigid column. The tubular member is made of resilient material, and the rigid column is made of composite material. Plural sorts of non-lead material, which are different in specific gravity, are mixed in such a manner that the composite material is adjusted to a target value of the specific gravity. The rigid columns are received in the resilient tubular members, and the resilient tubular members are inserted into the holes formed in the key through a press fitting.
The resilient tubular members are well fit in the holes. However, the rigid columns are merely held in the resilient tubular members by the agency of the resiliency of the resilient tubular members. In case where the rigid columns are finished smaller than the design drawing, the rigid columns are liable to be dropped out from the resilient tubular members.
Yet another key balance pieces are disclosed in Japan Patent Application laid-open No. 2001-154661. The key balance pieces disclosed in the Japan Patent Application laid-open is made of composite material, and small semi-spherical projections are formed on the outer surface of the key balance pieces. The composite material consists of plural sorts of non-lead metal and synthetic resin. The key balance pieces are inserted into the holes formed in the key through a press fitting, and the small projections are caught on the inner surfaces. Although most of the small projections are held in contact with the inner surfaces of the key, the semi-spherical projections have round contact surfaces, and are liable to slide on the inner surfaces of the key. In other words, the semi-spherical projections are hardly caught on the inner surfaces of the key. For this reason, the key balance pieces are liable to be dropped out.
Still another key balance piece is disclosed in Japan Patent Application laid-open No. 2001-175248. The key balance piece disclosed in the Japan Patent Application laid-open is made of composite material. Metal or alloy such as copper, brass iron and tungsten are mixed with fluid material such as thermosetting synthetic resin, thermoplastic synthetic resin, fusible alloy and adhesive compound in organic compound series. The metal or alloy is mixed with the fluid material, and the mixture is poured into cavities formed in a key. The mixture is solidified so that the key balance pieces are embedded in the key. However, it is not easy to fill the cavities with the mixture. If the mixture is too much, the key is contaminated with the residue. On the other hand, if the mixture is short, the key balance pieces are liable to be dropped out after the solidification.
Yet another key balance piece is disclosed in Japan Patent Application laid-open No. 2001-195056. The key balance piece disclosed in the Japan Patent Application laid-open consists of a tubular member and a rigid column. The tubular member is made of heat contracting synthetic resin, and the rigid column is made of non-lead metal. The rigid column is received in the tubular member, and the tubular member is inserted into the holes formed in the key through the press fitting. Although the tubular member are made of the heat contracting synthetic resin, the key balance piece consists of the combination of the tubular member and the rigid column as similar to the key balance piece disclosed in Japan Patent Application laid-open No. 2001-142454. For this reason, the key balance pieces are also unstable, and are liable to be dropped out.
The key balance pieces disclosed in those documents are embedded in the keys through the press fitting or solidification of fluid material. However, the key balance pieces are held in the holes or cavities by the agency of friction. For this reason, the key balance pieces are liable to be dropped out from the keys.
Key balancers are also used for the keys incorporated in the electronic keyboards, and are similar to those for the keys of the acoustic pianos. This means that the key balancers are liable to be dropped out from the keys.